1. Field of the Invention
The present invention relates to a signal processing method and signal processing apparatus able to reduce signal distortion.
2. Description of the Related Art
For example, an analog-digital mixed integrated circuit is promising as a system possessing both the high speed of an analog circuit and the flexibility of a digital circuit. In current analog circuits, high speed signal processing over 1 GHz or approaching 10 GHz is being sought. On the other hand, the power supply voltage has been steadily falling with each year and in the near future reportedly should become 1.0V or less. In an analog-digital mixed integrated circuit, the propagation of voltage fluctuations occurring due to the switching operations of a digital circuit through the semiconductor substrate to the analog circuit, that is, so-called “substrate noise”, has been considered a problem. However, if the signal amplification is large, the substrate noise is not that much of a problem. Rather, the effect of signal distortion due to the low power voltage has become remarkable.
To reduce the distortion of the output signal, it has been proposed to provide a plurality of copied signals offset from the given input signal by exactly a phase, separately perform the same processing, return the offset phases to their original states, then add them to obtain the output (see Eisse Mensink et al., “Distortion Cancellation by Polyphase Multipath Circuits” IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, Vol. 52, No. 9, September 2005). According to this method, it has been reported that distortion of the output due to the signal processing circuit of the mixer can be reduced. However, with this configuration, a circuit for offsetting a signal by exactly the phase is required. Realization of a circuit for precisely offsetting a signal by exactly the phase over a broad signal frequency band is difficult. Further, its application is limited to a mixer.